This invention relates to an improved alarm system for providing an alarm in response to the intrusion of at least one or a plurality of protective stations.
More specifically, this invention relates to an improved electronic burglar alarm system having a plurality of impedance branches which are connected to the legs of an electrical bridge circuit so that one or more of the legs can be made responsive to a varying condition resulting from the attempted intrusion at a protected station so as to change the impedance of that leg and to provide a visual or audible alarm.
In conventional burglar alarm systems, stations to be protected such as windows and doors of a building are wired in series connection to an electrical alarm system. A constant DC current is then applied to the wire and is used to hold a latching relay into an actuated position so that if an intrusion is made at any of the stations, the current will be momentarily interrupted so as to trip the latch relay and actuate the alarm system. Generally, the reconnection of the current at the station where the intrusion took place will not cause the latch relay to disconnect the alarm until a manual reset or timing switch turns the alarm system off. These conventional burglar alarm systems suffer from the disadvantage that persons familiar with the system can short circuit one or more of the stations and therefore bypass the alarm system to gain an intrusion into a building or home. The short circuiting of these conventional systems is generally performed by persons within the building structure such as employees, in order to gain illegal access to the building at a later time.
In order to overcome the disadvantages of these conventional systems, the applicant designed a DC impedance system so that each of the stations will operate at a predetermined impedance which is connected to a DC electrical bridge circuit. As soon as all of the stations are connected to the electrical bridge circuit, the bridge circuit can then be balanced so that the disturbance of any one station will upset the bridge and cause an audible or visual alarm to be actuated. Thus, anyone attempting to short circuit a station will immediately unbalance the bridge circuit and cause an alarm to be actuated. Such a system is shown in applicant's U.S. Pat. No. 3,609,739 which issued on Sept. 28, 1971.
With the ready accessability of more sophisticated equipment, burglars have been able to use electrical measuring instruments to determine the DC voltage connected across a door or station impedance, and attempt to circumvent the station impedance by using an electrical battery source having a potentiometer, so as to produce variable DC voltage. Thus, the voltage appearing across the impedance of a particular station can be measured and duplicated so as to bypass the system. Such an alteration of the system would generally take place when a building or office is unoccupied and the potential burglars have had an opportunity to gain partial access to the system and to a particular station and to measure the voltage across the impedance of that station.